Swimmers urged to stay out of Honolulu harbour after molasses spill

Health officials have warned people to stay out of the waters in Honolulu harbour after 1,400 tonnes of molasses leaked into the water.

A brown plume of the sweet, sticky liquid was spotted seeping into the harbour and Keehi Lagoon on Monday after a cargo ship, which was headed to the west coast of the United States, pulled out to sea.

Hawaii's department of health said on Tuesday a leak was discovered in a pipeline used to load molasses onto ships operated by Matson Navigation Co, an international ocean transport company.

The spill has killed hundreds of fish, which could potentially attract sharks.

So many fish had died by Thursday that the health department tripled the number of clean-up crews.

Department spokeswoman Janice Okubo said the crews removed hundreds of fish and were expected to remove thousands more in the coming weeks.

Everything that was underwater suffocated ... The whole bottom [of the ocean] was covered with fish, crabs, lobsters, worms, sea fans - anything that was down there was dead.

Dive shop owner Roger Smith

Ms Okubo said crews were monitoring molasses levels to help predict the spread and overall impact of the 840 kilolitre spill - which is roughly equivalent to one-third of an Olympic-sized swimming pool.

The department posted signs on beaches warning people to stay out of the water and not to consume any dead fish found in the area.

It said in a statement that while molasses was not directly harmful to people, it was "polluting the water, causing fish to die and could lead to an increase in predator species such as sharks, barracuda and eels".

It said that "an unusual growth in marine algae" and harmful bacteria was another environmental danger posed by the spill.

The brown plume was expected to remain visible for weeks while natural tides and currents slowly flush the area, the health department said.

Tourism officials said they were monitoring the situation but did not believe it would hurt Hawaii's primary source of income.

Roger Smith, a dive shop owner who went underwater on Wednesday to survey the damage, said it was unlike anything he had seen in 37 years of diving, with brown-tinted water and a layer of molasses coating the sea floor.

"Everything that was underwater suffocated," he said.

"Everything climbed out of its hole and the whole bottom was covered with fish, crabs, lobsters, worms, sea fans - anything that was down there was dead."

Matson acknowledged in a statement the spill was caused by a faulty molasses-loading pipe, which it said had been fixed.

It said molasses is a sugar product "that will dissipate on its own".

Matson said it regretted the incident and was working with authorities to take steps to ensure it did not happen again.

"We take our role as an environmental steward very seriously," the statement said.

"We have a long history in Honolulu Harbour and can assure all involved that this is a rare incident."